Environmental engineering (or life support center) was a section aboard Constitution-class starships where all life support activities on the ship were controlled and monitored. This section was located in the primary hull.
In 2268, when the Scalosians took over the Starship Enterprise, they installed a deep-freeze device in environmental engineering with the intention of putting the entire crew into suspended animation. (TOS: "Wink of an Eye")
In the alternate reality, this center was located aft of the warp nacelles. In 2259, during negotiations to return his crew to him, Khan Noonien Singh threatened to destroy this center if his request was denied. (Star Trek Into Darkness)